Insulation Resistance (IR) of electrical equipment refer to the resistance between conducting part and earth, expressed in mega ohms or in other words it is the ratio of applied voltage to the current between conducting part and frame of the machine for the fix time like one minute. This insulation resistance can also be measured between two circuits separated by insulation.
Insulation resistance is measured by Megger. Megger consists of a built in mega ohm meter and a D.C. generator. The generator is driven by hand or by a motor.
1.1 Constituent of Current in IR Measurement:
The total direct current Idc seen by the current coil of the Megger in insulation resistance measurement has following four components:
Idc = Ic + Ia + Ilc + Ils
where Idc = Total leakage current seen by the megger
Ic = Charging currents of capacitance
Ia = Dielectric absorption current
Ilc = Conduction leakage current through the insulation
Ils = Surface leakage current.
(Ic + Ia) flow for several seconds or minutes depending upon the size and capacitance of insulation. Their magnitude goes on reducing with time and after a few minutes (Ic + Ia) become zero.
The remaining two components (Ilc and Ils) are constant for a given applied voltage and are the true indicators of the insulation resistance. Wet insulation increases conduction currents through the insulation. Dirty surface of insulation increases the surface leakage currents.
Since the capacitive currents and absorption current reduce to zero after about 60 seconds, insulation resistance value after 60 seconds gives a better idea about quality of insulation.
The mega ohmmeter current time curve for relatively good insulation is shown in Fig. 1.1 and the component currents (leakage, capacitive, and absorption) that result in this behaviour are illustrated separately in
leakage component, shown in Fig. 1.1 (b), passes through or across the surface of the insulation. The magnitude of this leakage current depends on the resistance of the leakage paths and the value of the driving voltage; it is an Ohm’s law relationship. For good insulation, only a very small amount of leakage current occurs.
The capacitive component, shown in Fig. 1.1 (c), is caused by the capacitance between the wiring and the metal frame of the apparatus, and is typical of the charging current to a capacitor.
This component of test current starts high but drops rapidly, reaching almost zero in a very short time (five time constants). Because the duration of this current is very brief, it has very little effect on the indicate values of insulation resistance.
The absorption component, shown in Fig. 1.1 (d), converts electrical energy to stored energy in the form of a molecular strain in the insulation material. Although each molecule is electrically neutral (the positive charge is equal to the negative charge), its positive and negative charges form an electric dipole.
In the presence of an applied voltage, the positive charges are pulled toward the negative terminal, and the negative charges toward the positive terminal. Thus each electric dipole, not already aligned in the direction of the applied voltage, experiences a torque that tends to position it parallel to the line of action of the applied voltage.
This behaviour, called dielectric absorption, is a relatively slow process that may take many hours or days to complete. When the applied voltage is removed, and the wiring grounded, the molecules return slowly to their unstressed equilibrium position.
The dielectric-absorption characteristic of electrical insulation makes it both difficult and time – consuming to obtain an absolute measurement of insulation resistance. The insulation resistance indicated by the mega ohmmeter, at any instant of time, is the ratio of the mega ohm meter voltage to the mega ohmmeter test current (Ohm’s Law) :
Assuming a constant mega ohmmeter voltage, the indicated insulation resistance depends solely on the test current. If the test current is high, the indicated resistance will be low; if the test current is low, the indicated resistance will be high. Because the test current to relatively good insulation starts high and gradually decreases with time, as shown in Fig. 1.1 a, the indicated resistance starts low and increases with the continued application of test voltage.
Higher leakage currents through and across the surface of relatively poor insulation permit less accumulation of stored energy within the insulating material; this reduces the dielectric-absorption effect, causing both the current and resistance curves to flatten faster.
1.2 Equivalent Circuit of Dielectric:
A dielectric having power loss and leakage components, in the current through it, can be shown by an equivalent circuit as shown below for study and analysis.
The circuit has two branches in parallel. Capacitor ‘C’ & resistance ‘r1’ in series represents the dielectric absorption loss. The resistance ‘r2’ represents the leakage component.
In perfect dielectrics the value of ‘r1’, is zero & value of ‘r2’ is infinite. The values of C, r1 and r2 are not constant for all conditions, but are dependent on temperature, frequency, dielectric stress (applied voltage) & moisture. The principal sources of energy loss in dielectric are absorption & leakage. The absorption is more important.
1.3 MEASUREMENT OF INSULATION RESISTANCE
To measure the possible accurate insulation resistance following steps are to be performed in sequence, so the factors discussed earlier do not influence the measurement.
1.3.1. IR measurement with Megger:
Step – 1: Take care of the rated voltage for selection of megger before proceeding for measurement.
Step – 2: Ensure isolation perfectly.
Step – 3: Discharge the equipment using grounding / earthing lead for at least 15 min. First connect the lead to earth terminal and then remotely ground the equipment.
Step – 4: Disconnect the supply cables and any other connection of the equipment
Step – 5: Select the appropriate voltage of the megger. Megger voltage should be more than the rated voltage and below or equal to double of the rated voltage.
Step – 6: Make the connection for test as follows.
Step – 7: Crank the megger at rated speed if is manual and carry out the measurement as follows.
One terminal of megger is connected to conducting part. Other terminal is connected to earthed frame (or other conducting circuit), the rotor of the generator is driven by hand or by motor.
The reading Vdc/Idc read by the megger gives the insulation resistance value in mega ohm. The scale of Megger is graduated from zero to infinity in terms of mega ohms.
When D.C. voltage is applied to the insulation, initially the insulation draws capacitive charging currents (Ic) in addition to leakage current (Il). Thus the Megger reads.
1.3.2. Initial Megger Reading:
After continuous, application of D.C. voltage, the capacitance currents reduced and only leakage currents continue.
1.3.3. Megger Reading after 06 Seconds:
Hence the Megger reading after 60 sec. is higher than initial Megger reading.
For good dry machines insulation at the temperature between 15°C and 30°, the absorption coefficient (Kab) should be more than 1.3
For small and medium machines InR60 and InR15 give satisfactory and consistent results. For large machines, 10 Min. and 1 Min. readings are taken.
Insulation resistance measurement reveals quality of the transformers insulation and the degree of the dryness. The subsequent high voltage tests and commissioning can be avoided if insulation resistance is low.
Meggers (mega ohm meters) are available for D.C. voltages of 500, 1000 and 2500 V. In transformer practice, the 500 V mega ohm are used for measuring insulation resistance of the machines up to 60 kVA, and in all dry-type transformers of up to 1 kV, and the 2500 V meggers are employed for larger machines operating at voltages of 11, 33, 110, 220 kV and above.
The insulation resistance of a transformer is measured as follows:
- Between the windings collectively (i.e. with all the windings being connected together) and the grounded tank (ground).
- Between each winding and the tank, the rest of the windings being grounded.
- For a two-winding transformer the following three measurements are to be taken:
– Between LV winding and tank, the HV winding being grounded;
– Between the HV winding and tank, the LV winding being grounded;
– Between the LV/HV windings collectively, and the tank grounded.
- For a three winding transformer five measurements are necessary;
– Between the HV winding and the tank, the LV1 and LV2 windings being grounded;
– Between the LV1 winding and tank, the HV and LV2 windings being grounded;
– Between LV2 winding and tank, the HV and LV1 windings being grounded;
– Between the HV and LV1 windings collectively, and the tank, the LV2 winding being grounded.
When measuring the insulation resistance of a current carrying component part relative to ground, the following procedure can be employed. Two flexible well-insulated conductors should be connected to the line and earth terminals of a Magger. The free ends of conductors must have metal terminals (probes) with insulated handle. The probe marked “ground” should
be connected to the transformer tank (that must be grounded) and the other probe, to a current carrying bar or lead connected to the winding under test and the Megger should be cranked at a speed indicated in this certificate (usually 180 r.p.m.) 60 seconds after the start of the cranking the insulation resistance should be read on the megger scale. The value thus obtained is called 60 second insulation resistance and is designated R60. Ten minute reading is taken for large transformer.
When measuring the insulation resistance between different windings, the Megger probes should be connected directly to the windings.
Step 8 : Discharge the equipment for megger time.
Step 9 : Note down the winding temperature for correction, comparison and analysis.
Step 10 : Reconnect all wires and cables disconnected earlier.